In internal combustion piston engines the rings are formed, in general, of a metal base to which there is applied at least one layer of coating which comes into contact with the wall of the cylinder. As an example, there may be cited a base of cast iron or of stainless steel, utilised as a function of the cost and/or of the type of stress to which the ring will be subjected.
The role of the layer of coating is very important because of the fact that it seeks to confer properties on the ring of low sliding friction and high spalling resistance, hardness and toughness. Nevertheless, it must be noted that some of these properties conflict and an increase in one thereof means a loss of performance in another, because whereof the type of coating to be utilised will be selected as a function of the operational profile of the engine and of the stresses whereto the rings will be submitted during their working life.
In an internal combustion engine the compression ring, or first groove ring, is that which experiences most stresses by virtue of being in direct contact with the superheated gases of the burning compressed air mixture. Consequently, by virtue of the very location thereof, they are continually exposed to high temperatures and to aggressive byproducts of the combustion.
A series of coatings has been proposed by those skilled in the art, each thereof seeking to improve the properties desired, increasing the strength of the piston rings. Several prior art refers to the ceramic coatings of chromium nitride in the form of CrN and/or Cr2N, very efficient and technically interesting because of the fact that they balance in a very competent manner the properties of absorption of internal stress, high hardness and low thickness, greatly reducing the occurrence of spalling.
In the great majority of the applications the ceramic coating of chromium nitride is applied by the PVD process, according whereby the material to be deposited as a coating is vaporised from a solid source in the form of atoms or molecules by means of low-pressure gases (or plasma) in the direction of the piston ring, whereon it condenses. The formation of the coating on the substrate depends on the materials vaporised and on the material of the substrate, on the conditions of the surface, on the energy available (temperature and ion bombardment), and on the atmosphere (chemical reactions, reactive deposition, etc).
The PVD process presents some interesting advantages for the production of tribological coatings such as, for example, (i) the possibility of operating at very low working pressures thereby rendering possible the synthesis of materials of high purity; (ii) improvement in the adhesion of the coating to the substrate because of the possibility of “cleaning” the surface of the substrates by means of the ion bombardment; (iii) uniform thickness of the coating; (iv) control over the crystalline structure of the coating; and (v) relatively low deposition temperatures. As additional advantages ion bombardment promotes epitaxial growth and improves the atomic mobility, and normally no effluents or pollutants are utilised because of the fact that in the majority of cases toxic products or solutions are not involved.
In the specific case of engines of the high-speed diesel type, because of the high combustion pressures and the high rotational speeds which they reach, the rings are very stressed, in particular the compression rings. For that reason, compression rings are in general specified having a base constituted of stainless steel the totality thereof being nitrided, having a wear-resistant coating applied to the external face, a posteriori, by the PVD process.
This type of ring is known for its high mechanical strength and by the high price thereof, nevertheless in some applications even they do not manage to prevent the emergence and propagation of cracks, not only in the coating but in the entire ring, which may lead to a catastrophic failure entailing the disassembly of the engine.
In these situations, cracks are initiated at the upper and lower chamfers, being respectively the edges joining the lateral face to the upper face and to the lower face. In some cases the cracks ends up by propagating throughout the cross-section of the ring, rupturing the part and requiring the complete disassembly of the engine.
With the objective of solving this difficulty, the U.S. Pat. No. 6,508,473 reveals a piston ring based on stainless steel nitrided on the upper and lower faces thereof. The nitrided faces have a hardness of 700 HV. The ring comprises furthermore a film of hard coating on the external face thereof in contact with the wall of the liner/cylinder, spaced from 0.0001 mm to 0.3 mm from the upper and lower faces. According to the patent, this spacing, although minimal, prevents the propagation of cracks up to a reasonable limit of utilisation, however in regimes of severe utilisation this technique does not guarantee the strength of the ring.
Up to the present time a piston ring, especially a compression or first groove ring, has not been developed which manages to attain the standards of durability demanded in severe applications, such as those demanded by the diesel engines operating at high feed pressures and at high rotational speeds.